Ocular Anatomy: Cornea

Cornea Functions

Transparent

Major refractive surface

Mechanical Strength

Barrier

Protection

How is the cornea transparent

- avascular

- Structural arrangement

- hydration

How does the cornea offer Mechanical Strength

Collagen!

maintains ocular contour

arrangement maintains transparency

How is the cornea a barrier

Part if the Ocular defense system along with conjunctiva and tears

How does the cornea protect the eye?

Highly innervated!

reflex blinking and tearing

sensory protective mechanisms

What is on the anterior side of the cornea?

tear film

What is on the posterior side of the cornea?

aqueous humor

What structure is adjacent to the cornea?

Limbus

Characteristics of the anterior surface

convex

aspheric

prolate (elongated)

the anterior cornea is steepest in the

center

the anterior cornea flattens in the

periphery

The posterioir surface is ____ than the anterioir surface

steeper

Anterior diameter is longer _____ than ______

horizontally 11.7mm

vertically 10.6mm

The posterior diameter is

sphere!

11.7mm

What is it called to measure the thickness of the cornea

pachymetry

Where is the cornea thicker

periphery

What is the average thickness

544 +/- 34 um

Thickness of the cornea increases with

increased hydration

When is the cornea thicker?

In the morning

Eyelids are closed when we sleep

tears are isotonic with cornea

no fluid pulled out

When is the cornea thinner?

Day

Eyelids are open, tears evaporate, tears hypertonic

Epithelium of Cornea

5-7 layers of non-keratinized stratified squamous epithelium

uniform arrangement of cells

superficial cells exfoliate so much replenish

Epithelium of Cornea is (vascular or avascular)

Avascular!

Epithelium of Cornea is continuous with

conjunctiva epithelium (at the limbus)

Epithelium: Squamous layer surface modifications

Gylcocalyx

Microplicae and Microvilli

Tight junctions

Glycocalyx

glycoproteins and glycolipids and mucin

interacts with mucin layer of tears to create a smooth surface and maintain hydrophilic properties

enhance tear stability

Microplicae and microvilli

increase surface area

anchor tears

Tight junctions

anterior corneal Barrier !

Wing cells

beneath single layer of squaomous cells

2-3 layers

Wing cells

highly innervated cells that differentiate into squamous cells do not divide

Basal cells

single layer at the base membranse

mitotically active (wing cells-->squamous cell)

transit amplifying layer!

Secretes basal lamina !!!!!!!!!

hemidesmosomes are found

on the basal surface of epithelial cells

SEE NOTES

Dendritic langheran cells

APCS

many in periphery

usually absent in central cornea

(biodefense)

Epithelial regeneration

Limbal stem cells --> transient amplifying cells --> new basal epithelial cells --> mitosis --> wing cells --> differentiate into squamous cells

squamous cells are shed

7-14 days

DRAW THIS

Bowman's layer

randomly arranged small diameter collagen (22nm)

acellular

provides shearing strength

Barrier (prevents large substances from entering due to interweaving)

strongly attatched to stroma

STROMA

Stroma contains

collagenous protein

cells - keratocytes

grund substance - non-collagenous protein

collagen

Lamellae

Type 1 collagen fibers are uniform in diameter

due to proteoglycNA

Lamellae

sheets of collagen fibers

Alternative arrangement of collagen fibers in adjacent lamellae

Fibers in lamellae are more disorganized

anteriorly and peripherally

Fibers in lamellae are more organized

posteriorly and centrally

Lamellae characteristics

extend limbus to limbus

tensile strength

shape stability

transparency

How do collagen fibers change at the limbus

run circumferentially around the cornea at the limbus

maintains the curvature of the cornea

Keratocytes

corneal fibroblasts (fibrocytes)

Keratocytes are connected by

tight junctions

gap junctions -- functional syncytium

Majority of keratocytes are found

sandwiched between lamellae

When keratocytes are activated

can migrate to wound margins and synthesize collagen, GAGS, and MMPS

Keratocytes can also synthesize

intracellular cyrstallins

Non collagenous proteins of the stroma

Proteoglycans

Glycoproteins

Proteoglycans

protein core with sulfated glycosaminoglycans (GAG) side chains

highly negatively charged - attract Na+ and water

(Repel each other) surround collagen fibrils to create uniform spacing between fibrils

Stroma transparency is due to

regularity of collagen fibers thickness (small diameter)

consistency of distance between fibers

Lamellar arrangement in stroma

Anterior Stroma

1/3

Collagen fibers are thicker than Bowman's layer (but thinner than posterior)

Lamellae cross at oblique angles

lamellae are more disorganized than posterior

contributes to shear resistance

less tendency to accumulate water than posterior

Posterior Stroma

2/3

collagen fibers and lamellae thicker

precise arrangement of lamellae (orthogonal)

provides tensile strength

greater tendency to accumulate water than anterior stroma

DESCEMT's MEMNRANE

Descemet's Membrane

Basal Lamina of endothelial cells

thickens with age

no elastic fibers but elastoid properties

weakly attatched to stroma

thickened at limbus to form schwalbles line (ring)

Which layer develops before borth

Anterioir layer (banded)

Which layer develops after birth

posterior

this layer increases thickness with age

Descemets layer funciton

provides a barrier to infection, inflammation, and perforating injury

can withstand normal IOP

ENDOTHELIUM

endothelium

mosaic single layer of cells on back if cornea (hexagonal cells)

metabolically active --> synthesize descents membrane

The endothelial cells are connect by

leaky tight junctions

allow diffusion of glucose and nutrients from aqueous

controls hydration of stoma

Why are they leaky

they dont have as many protein complexes

apical surface is twoard

aqueous (posterioir)

endothelial cells have

club like microvilli

mitochondria - metabolically active

overlapping marginal folds

glycocalyx

as we get older we lose

what happens

endothelial cells

cells change size and shape to fill the gap

Cell density of endothelium

decreases

when do we get concerned

400-700 cells/mm2

density is greatest in peripheral cornea

INNERVATION

the cornea is the most densely

innervated tissue of the body

long posterioir cilliary nerve --> noscilliary nerve --?m pothalmic divison of trigeminal nerve

SEE NOTES

Lumbal plexus

70-80 large nerve trunks

radiate into the cornea

protective mechanism

Free nerve endings

pass between basal cells and terminate among epithelial cells

lose schwann cells

Greatest sensitivity is in

the center!

Trophic effect

Promotes growth and health of target tissue

Corneal reflex blink

protect the surface of the eye

muscles that open and close the eyelid is activated

Tear reflex

salvitory nucleus SEE NOTES

Blood supply t the cornea

opthalmalmic

muscular

anterior ciliary

Superifical and depp episcleral

SEE NOTES FOR BLOOD SUPPLY